Metamorphism
- mineralogic or textural changes in a rock in the solid state as
a result of increasing pressure and/or temperature, or as a result of emplacement
of a rock type into a chemical environment in which its constituent minerals
are not stable.Minerals in rocks
undergoing metamorphism can:
1)be
recrystallized without any reactions occurring,
for
example, fine-grained quartz recrystallizing into larger grains of quartz.
Quartz
arenite (a type of sandstone and hence a sedimentary rock) can be metamorphosed
to produce quartzite (a metamorphic rock)
2)they
can react to form other minerals with no change in bulk rock chemistry
,
for
example, polymorphic transitions, as in:
kyanite
(Al2SiO5)--> andalusite (Al2SiO5),
or
coupled solid-solid reactions, as in:
2
forsterite + anorthite --> 2 enstatite + diopside + spinel;
2
Mg2SiO4 + CaAl2Si2O8
-> 2 MgSiO3 + CaMgSi2O6 + MgAl2O4
3)they
can react with other substances added to the system to form new minerals,
for
example, minerals reacting with fluids containing dissolved cations, as
in:
3
kyanite + 3 quartz + 2K+ + 3 H2O --> 2 muscovite
+ 2H+;
3
Al2SiO5 + 3SiO2 + 2K+ + 3 H2O
--> 2 KAl2(AlSi3O10)(OH)2 +
2H+
4)they
can break down or react with other minerals to release fluids and form
new minerals,
for
example,
calcite
---> lime + carbon dioxide
CaCO3
--> CaO +CO2;
or,
calcite + quartz ---> wollastonite + carbon dioxide
CaCO3
+ SiO2 --> CaSiO3
+ CO2;
or,
iron-chlorite + quartz ---> almandine + H2O.
Onset
of metamorphism: This
is a “fuzzy” boundary with diagenesis.Onset
of metamorphism is indicated by the presence of a mineral that would not
normally be found in an igneous rock or in a sedimentary rock.The
temperature of the onset of metamorphism can range from 150-200°C
up to 350°C,
depending on rock type (that is, the minerals the rock contains).
Types
of Metamorphism:
Burial
Metamorphism
Regional
Metamorphism
High-Pressure,
Low-Temperature Metamorphism
(Blueschist Facies Metamorphism)
Contact
Metamorphism
Shock
Metamorphism (Impact
Metamorphism)
Metamorphic
Recrystallization
Increased
temperature in the absence of shear stresses will generally increase grain
size.
This
is the result of processes that tend toward a minimization of energy in
a system.
Annealing
is the process of recrystallization such that average grain size is increased
and a
polygonal
texture (in monomineralic rocks) is approached.
In
rocks without platy or prismatic minerals, annealing produces a granoblastic
texture.
Terms:Protolith
- the igneous or sedimentary rock that was the precursor the metamorphic
rock
Meta
- as a prefix, indicates a metamorphosed rock; for example, metagabbro.
The
majority of metamorphic rocks fall into 3 broad categories based on protolith
chemistry:
Aluminous
clastic sedimentary rocks (like clay-rich mudrocks or feldspar-rich sandstones)
Calcareous
rocks (like limestone, dolostone, or marl)
Mafic
or intermediate volcanic and pyroclastic rocks (like basalt and andesite)
These
are the most common protoliths because they are the must abundant rock
types
found
in the tectonic environments that become incorporated into metamorphic
and
orogenic
belts.
Mudrocks
---> Metapelites
Slate
Phyllite
Schist
Gneiss
Migmatite
Calcareous
Rocks --->
Marble
Calcsilicates
Mafic
and Ultramafic Rocks --->
Greenstones
or Greenschists - mafic schists
Amphibolite
Mafic
Gneisses
Other
Metamorphic Rock Types
Quartzite
Eclogite
Hornfels
Skarn
Serpentinite
Granulite
Mylonite
Metamorphic
Textures and Structures
Metamorphic
minerals are called blasts; textural terms often end with the suffix
-blastic.
Granoblastic-metamorphic
rock composed of equidimensional mineral grains (equigranular).
Porphyroblasts-sub-
to euhedral crystals that are distinctly larger than the matrix.
Poikioblasts-porphyroblasts
that contain numerous small inclusions of matrix minerals.
Foliation-general
term for pervasive planar structure
Slaty
Cleavage-a common type of rock
cleavage (planes along which a rock breaks most easily).
closely-spaced
planes along which a slate breaks; may or may not be parallel to
bedding
planes of the protolith.
Schistosity-a
type of foliation wherein platy minerals can be identified with the unaided
eye
or
with a hand lens.
is
no propensity to break parallel to the foliations.Mineral
layers are typically
on
the millimeter to centimeter scale.
Mylonitic-fine-grained,
inequigranular rock formed by deformation (as in crushing in a
shear
zone/fault).
Augen-lenticular
or eye-shaped porphyroclasts.
Porphyroclast-large,
shear-bounded grain in a finer-grained, crushed matrix
Isograds-Concept
developed by Barrow (1890's) working in metapelites in Scotland.
isograd-a
line on a map that separates adjacent metamorphic zones.
delineated
by the first appearance of a critical index mineral.
Applied
to varying grade within a constant lithology.
metamorphic
zone-a mappable part of a metamorphic
body where rocks are of a similar grade.
index
mineral-a mineral which appears in
a specific metamorphic zone but not in the
metamorphic
zone of lower grade.
metamorphic
grade- a set of P, T, X conditions under which a certain metamorphic mineral
assemblage
forms; low-grade generally means lower T, P conditions whereas
high-grade
generally means higher T, P conditions.
Prograde
metamorphism - sequence of metamorphism whereby grade increases throughout
the
process;
for example during burial and heating.
Retrograde
metamorphism - sequence of metamorphism whereby grade decreases throughout
the
process;
for example during uplift and unroofing.
Metamorphic
Facies - Concept
developed by Eskola (1915) based on work on contact metamorphosed
rocks
in SW Finland.
Defined
as the whole set of mineral assemblages that could occur if protoliths
of varying
chemistry
(spatially associated rock types) were all metamorphosed together.
SEE
PAGE 588 P-T graph with facies names.
The
most common facies are:
Zeolite
Greenschist
Amphibolite
Granulite
Blueschist
(also called glaucophane-lawsonite)
Eclogite